Method of treating articles requiring annealing



May 29, 1956 T. c. BURNHAM METHOD OF TREATING ARTICLES REQUIRING ANNEALING Filed March 25, 1951 56 BLOWER, 2

United States Patent Office.

METHOD OF TREATlNG ARTICLES REQUIRING ANNEALING Thomas C. Burnham, San Diego, Calif, assignor to Rohr Aircraft Corporation, Chula Vista, Calif., a corporation of California Application March 23, 1951, Serial No. 217,257

6 Claims. (Cl. 148-615) This invention relates to a method for treating metal articles which require annealing. It is an object of the invention to provide a method which will prevent flaws on the surface of annealed metal articles after the scale incident to the annealing process is removed.

Another object is to provide a method for eliminating the deleterious effect which foreign material on the surface of certain metal articles has on their appearance, "after they are annealed and the annealing scale removed.

Still another object is to provide a method for facilitating the removal of scale from stainless steel articles which require annealing without detracting from their appearance after removal of the annealing scale.

Further objects of the invention will become apparent as the description thereof proceeds. For a better understanding of the invention reference is made to the accompanying drawings, in which:

Figure 1 is a schematic view, partly in section, of apparatus used for coating the articles;

Figure 2 is a schematic view, partly in section, of a furnace used for annealing the articles;

Figure 3 is a schematic view of apparatus used for cooling the annealed articles;

Figure 4 is a schematic view, partly in section, of apparatus used for descaling the articles;

Figure 5 is a schematic view of apparatus for washing the articles, one of which is shown in cross section, and

Figure 6 is a sectional view of one of the articles.

In the manufacture of articles from corrosion and heatresistant steels and other alloys, drawing and forming operations are often performed which harden the ma terial and also induce large internal stresses in it. It is often found necessary to anneal such articles in order to relieve them of the internal stresses and condition them to be more easily formed in later operations. After the articles are treated to remove the scale formed on the surface during annealing, it is found that the surface does not have a uniform appearance but that some areas are more or less discolored and constitute blemishes on the annealed article. Any such blemishes constitute flaws in the article and adversely affect its saleability.

It has been found that such flaws result from two causes. The first is that the sheet stock from which the articles are cut and formed has a thin layer or shell on its surface whichis harder than the material underneath and this shell is broken at many places where the article is bent during the forming operations. When the article is annealed, it is believed that oxides of somewhat different composition are formed at the areas where the shell is broken, then over the rest of the surface. When these oxides are later removed by a descaling solution, blemishes result from the differential action of the solution on the different oxides.

The second cause is the foreign material deposited on the surface of the articles during their manufacturing and handling. Such foreign material may cover the entire surface of an article or only some portions of the surface and may consist of dust particles raised from floor time to drain.

and are ready for annealing.

2,748,037 Patented May"29,:1956

2 sweeping, particles produced during grinding operations and floating about in the air, oil film and other material varying with the local surroundings.

It is believed that the presence of such foreign material on articles requiring annealing, causes the formation of different oxides in the regions covered by foreign material than in the clean areas and that after treatment by a descaling solution to dissolve the oxides and remove the scale, blemishes appear on the surface of .the article at the areas where the foreign material was. It has been found by coating the articles with certain materials prior to their annealing that during the anneal heating the applied coating reacts with the foreign material .and the surface of the article to form compounds which are entirely removed by the treatment later used to remove the annealing scale. A smooth, uniform surface texture is thus secured which has the same color shade throughout and is free of blemishes.

In the embodiment of the invention illustrated, each article 10 to be treated is shown of hemispherical shape and having exterior and interior curved surfaces 11, 12 (see Figures 5-6). Each article has been previously blanked out from flat sheet metal and formed in a press to the desired shape. During this working of the metal, it is hardened and internal stresses set up in it, as above noted. It will be understood that this invention is not limited to the processing of articles of the shape illustrated but they may be of any shape whatsoever. Also, while each article is shown as a single piece of metal, the invention is not limited to such construction and an article may be formed of two or more pieces which are secured together by any known securing means. The foreign material above referred to is shown as a continuous layer 13 (see Figure 6) which covers the inside and outside of the article, however, this layer may be discontinuous and cover only a limited area or areas.

Coating The application of.a coating :to the articles, which is an import-ant feature of the invention, is effected in a tank 14 (Figure 1) containing a coating solution 15. The tank is heated by a gas *burner to maintain the solution at a temperature of preferably F., the gas being supplied through a pipe 17 and regulating valve 18. The articles to be coated are placed in a basket 19 whose bottom and sides are made of Wire, perforated metal or other material which is chemically inert to solution 15. The articles are not stacked up but alranged so that the solution has free access to every part of each article being coated. After being loaded, basket 19 is attached to a cable 20 and lowered into solution 15 by a power-driven winch 21. After thorough'wetting by the warm coating solution, basket 19 is raised by winch 21 to remove the articles and they are suspended above tank 14 for a short They are then removed from the basket Typical austenitic steels on which my process has proved effectivea-re listed below, the components being given in per cent by weight.

Sample No. 1 Sample No. 2 Sample No. 3

Chromium. 24 to 26 17 to 19 18 to 22 19 to 22 12 to 14 8 to 10 2.0 maximum. 1.5 maximum.

.08 maximum.

. 2.0 maximum.

0.12 maximum 0.50 maximum 2.0 to 2.5 1.00 to 1.75. opper 0.50 maximum .50 maximum. .50 maximum. Tungsten 0 maximum... 0 1.00 to 1.75. Minor Gonstituents.. .90 maximum. 1.40.

Balance Iron.

Balance Iron.

.36 maximum.

One corrosion and heat-resistant alloy which is not austenitic and which has been successfully treated is:

Chromium 20%, nickel 20%, cobalt 20%, molybdenum 3%, tungsten 2%, columbium 1%, minor constituents 1.05%, balance iron.

Coating solutions which have proved effective have the following percentage compositions by weight:

San'tple N0. -1.-'Sodium nitrate 43.5%, isopropyl alcohol 1%, sodium alkyl aryl sulfonate .06%, water 55.44%. I

Sample No. 2.--Sodium carbonate 25%, trisodium phosphate 5%, sodium alkyl aryl sulfonate 0.25%, isopropyl alcohol 5%, water 64.75%.

Sample No. 3.Trisodium phosphate 1%, phosphoric acid sodium alkyl aryl sulfonate 0.25%, isopropyl alcohol 5%, water 83.75%.

The sodium alkyl aryl sulfonate acts as a detergent and also to decrease the surface tension of the liquid thereby promoting the wetting of the surfaces of the articles being coated. Other suitable surface tension-reducing agents are isopropyl naphthalene sodium sulfonate and dioctyl sodium sulfosuccinate. The isopropyl alcohol promotes the rapid drying of the coating on the articles after their removal from the coating solution, and it will be understood that alcohols of diflierent compositions may be used instead.

Annealing Referring to Figure 2, the articles 10, after coating, are placed on a steel tray 22 which is immediately put in an annealing furnace 23. Dust and other particles floating about in the air of the factory are thus prevented from settling on them, as would be the case if they were permitted to remain exposed for a long time. The articles are not stacked together on tray 22 but are arranged so that the furnace gases circulate freely around them. At its front end, furnace 23 is provided with a door 24, slidable in a vertical guideway 25, the door being shown in its closed position. The interior of the furnace is provided with a gas burner 26 to which natural gas is supplied through a pipe 27 and control valve 28, and air under pressure through a pipe 30 and control valve 29. By using natural gas consisting chiefly of methane and small amounts of other hydrocarbons, it is easy to regulate the valves 28 and 29 so as to maintain a reducing at mosphere in the furnace. This may be checked by analyzing the burned gases issuing from the chimney 31, a carbon monoxide content of 3% to 4% being satisfactory. The temperature in the furnace is controlled in accordance with the composition of the alloy being annealed, a temperature of about 2,050 F. being suitable for austenitic steels of the types above described. While the furnace illustrated is heated by gas, it will be understood that other sources of heat may be used instead. The heating is continued until the articles are fully annealcd; in the case of the steels above described, a period of 2 to 3 minutes for each .010 inch of stock thickness being suitable.

It is thought that the alkali metal salt contained in the coating, reacts at the annealing temperature with the ash of the foreign material on the articles and with the oxides of their alloying elements to form compounds which are soluble in a descaling solution of the type described later herein.

Cooling After annealing, the door 24 of the furnace is raised and tray 22 with the articles 10 thereon, removed from the furnace. The loaded tray is placed on a grating 32 which forms the top of a carriage 33 provided with supporting rollers 34. Carriage 33 is rolled under a hood 35 to which a current of air at room temperature is supplied by a power operated blower or fan 36. By this means the articles are rapidly cooled to room temperature and made ready for descaling.

4 Descaling The descaling is eflected by an acid solution 37 contained in a tank 38 which is open to the atmosphere. Tank 38 comprises a steel vessel 39 lined with an acid-resistant synthetic rubber lining 40 which is in turn covered with carbon brick as indicated at 41. The solution 37 is heated to a temperature of between F. and F. by any suitable means, as by live steam passing through a heating coil or pipe 42 which passes through tank 38, but electricity or other heat source may be used instead of steam. The annealed articles 10 are placed in a basket 43 made of wire or perforated metal which is not affected by solution 37 and the basket attached to a cable 44 wound about a power operated winch 45. The winch is operated to lower the basket and articles 10 into the descale solution where they are permitted to soak until the scale is dissolved. The soaking period varies from 10 to 30 minutes or longer according to the nature of the scale and chemical composition of the articles. While the articles are soaking, solution 37 is kept well agitated by jets of air issuing from the lower ends of two or more pipes 46 to which air under pressure is supplied by a supply pipe 47. This agitation keeps the sediment formed in the solution from settling on portions of the articles and thus preventing free access of the solution to such portions, a result which would interfere with uniform and rapid descaling. It also aids in dissolving and mixing chemicals added to keep the solution up to strength.

A descale solution found suitable for dissolving the scale of corrosion and heat-resistant stainless steels, and other chrome-nickel alloys has the composition by weight; 10 per cent ferric sulphate, 3.3 per cent hydrofluoric acid (free acid) and 86.7 per cent water. It is desirable to maintain a 3 to 1 ratio of ferric sulphate to hydrofluoric acid, therefore the solution should be tested at intervals and chemicals added to maintain the indicated strength 7 and composition.

W ash irtg -which rinses the acid from articles 10 and removes a considerable portion of the loose scale. The basket and articles are then raised by winch 45 out of the rinse water and the' articles removed from the basket. Usually some loose scale still adheres to the articles and to remove this entirely, each article is individually washed by a combined air and water spray issuing from a movable nozzle 48 (see Figure 5). Water is supplied to nozzle 48 through a flexible hose 49 and control valve 50, and compressed air supplied through a flexible hose 51 and control valve 52. After washing, the surface of the article has a uniform texture and color and is free of flaws such as discolorations and blemishes.

My invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The present embodiment, as herein set forth, is therefore to be considered in all respects as illustrative and not restrictive, the scope of my invention being indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.

Having thus described my invention, what I claim as new and useful and desire to secure by Letters Patent is:

1. The method of annealing producing a smooth surface of substantially uniform texture and having the natural color of the alloy on a plurality of similar stainless steel articles formed to a curved shape and having internal stresses due to the forming, comprising the steps of wetting the entire articles simultaneously with an aqueous solution of a surface tension-reducing agent and a salt of the group consisting of sodium nitrate, sodium carbonate and trisodium phosphate to form a coating on the articles; supporting the articles in spaced apart positions in a reducing atmosphere so that the entire coated surfaces of the articles are exposed to the reducing amosphere and simultaneously heating the coated articles to an annealing temperature; cooling the annealed articles simultaneously; immersing the articles simultaneously in a hot aqueous solution of ferric sulphate and hydrofluoric acid to dissolve the scale on the articles; and washing the articles to remove the dissolved scale.

2. The method of claim 1 comprising the additional step of forcing one or more streams of air into the hot aqueous solution while the articles are immersed therein.

3. The method of annealing curved stainless steel articles and removing a film of foreign matter from the surface of a plurality of similar stainless steel articles formed to a curved shape and having internal stresses due to the forming; comprising the steps of soaking the articles with the film thereon in an aqueous solution of a surface tension reducing agent and a salt of the group consisting of sodium nitrate, sodium carbonate and tri sodium phosphate to form a thin coating on the articles; supporting the coated articles in spaced apart positions in a reducing atmosphere so that the entire coated surfaces of the articles are exposed to the reducing atmosphere and simultaneously heating the coated articles to an annealing temperature; cooling the annealed articles simultaneously; soaking the annealed articles simultaneously in a moving aqueous solution of ferric sulphate and hydrofluoric acid to dissolve the compounds formed by the film of foreign matter and the annealing scale with the coating on the articles; and washing the articles with water to remove the dissolved scale and permit the color of the alloy composing the articles to be seen.

4. The method of annealing and cleaning a plurality of curved stainless steel articles which are internally stressed and have an adherent dirt film on the surface thereof, comprising the steps of arranging the articles in spaced apart positions on a movable carrier so that each article is out of contact with adjacent articles; heating and aqueous solution of a surface tension-reducing agent and sodium nitrate; lowering the carrier with the separated articles thereon into the heated solution to provide a coating on the surface of each article; raising the carrier and the articles thereon out of the solution; supporting the carrier over the solution to permit excess coating on the articles to drain into the solution; removing the coated articles from the carrier; supporting the coated articles in spaced apart regions of a reducing atmosphere so that the entire surfaces of the articles are exposed to the reducing atmosphere and simultaneously heating the coated articles to an annealing temperature; cooling the anne; articles simultaneously; and soaking the cooled articles simultaneously in a moving aqueous solution of ferric sulphate and hydrofluoric acid to dissolve the compounds formed by the iilm of foreign matter and the annealing scale with the coating on the articles.

5. The method of preparing a plurality of curved stainless steel articles which are internally stressed and have an adherent dirt film on their surface for forming, which comprises the steps of arranging the articles in spaced apart positions on a movable carrier; heating an aqueous solution of sodium carbonate about 25%, trisodium phosphate about 5%, alcohol about 5%, balance water to a temperature of the order of F; lowering the carrier with the separated articles thereon into the heated solution to provide a coating covering the surface of each article; raising the carrier and the articles thereon out of the solution; supporting the carrier and articles above the solution to permit excess coating on the articles to drain into the warm solution, removing the articles from the carrier; and annealing the coated articles.

6. The method of preparing a plurality of curved stainless steel articles which are internally stressed and have an adherent dirt film on their surfaces for forming, which comprises the steps of arranging the articles in spaced apart positions on a movable carrier; heating an aqueous solution of trisodium phosphate about 1%, phosphoric acid about 10%, alcohol about 5%, balance water to a temperature of the order of 80 F.; lowering the carrier with the articles thereon into the heated solution to provide a coating covering the surface of each article; raising the carrier and the articles thereon out of the solution; supporting the carrier and articles above the solution to permit excess coating on the articles to drain into the warm solution; removing the articles from the carrier; and annealing the coated articles.

References Cited in the file of this patent UNITED STATES PATENTS 1,901,039 Owens Mar. 14, 1933 2,172,422 Uhlig Sept. 12, 1939 2,227,945 Neilson Jan. 7, 1941 2,230,319 Canzler Feb. 4, 1941 2,389,838 Bromberg Nov. 27, 1945 OTHER REFERENCES Materials and Methods, pp. 74, 75, January 1951. 

1. THE METHOD OF ANNEALING PRODUCING A SMOOTH SURFACE OF SUBSTANTIALLY UNIFORM TEXTURE AND HAVING THE NATURAL COLOR OF THE ALLOY ON A PLURALITY OF SIMILAR STAINLESS STEEL ARTICLES FORMED TO A CURVED SHAPE AND HAVING INTERNAL STRESSES DUE TO THE FORMING, COMPRISING THE STEPS OF WETTING THE ENTIRE ARTICLES SIMULTANEOUSLY WITH AN AQUEOUS SOLUTION OF A SURFACE TENSION-REDUCING AGENT AND A SALT OF THE GROUP CONSISTING OF SODIUM NITRATE, SODIUM CARBONATE AND TRISODIUM PHOSPHATE TO FORM A COATING ON THE ARTICLES; SUPPORTING THE ARTICLES IN SPACED APART POSITIONS IN A REDUCING ATMOSPHERE SO THAT THE ENTIRE COATED SURFACES OF THE ARTICLES ARE EXPOSED TO THE REDUCING ATMOSPHERE AND SIMULTANEOUSLY HEATING THE COATED ARTICLES TO AN ANNEALING TEMPERATURE; COOLING THE ANNEALED ARTICLES SIMULTANEOUSLY; IMMERSING THE ARTICLES SIMULTANEOUSLY IN A HOT AQUEOUS SOLUTION OF FERRIC SULPHATE AND HYDROFLUORIC ACID TO DISSOLVE THE SCALE ON THE ARTICLES; AND WASHING THE ARTICLES TO REMOVE THE DISSOLVED SCALE. 